1. Field of the Invention
The present invention relates to an image data encoding/decoding technique.
2. Description of the Related Art
Recent spread of personal computers and mobile terminals has led to broad use of digital data communication (data communication) via the Internet. Digital data used in data communication include images. An advantage of digital images is that, for example, copying, searching, and various kinds of image processing are relatively easy. Hence, a lot of people handle digital images.
For efficient use of digital images, image compression to reduce the file size is indispensable. Typical techniques are JPEG and PNG. Image compression may distort an image. Techniques of suppressing image quality degradation have been researched and developed. Techniques of attaining not only the primary function of reducing the file size but also attaining various other functions have also been developed. For example, JPEG 2000 enables encoded data generation by scalably decoding image data gradually from a low resolution up to its original resolution.
Techniques having an image quality improving function in addition to the above-described resolution scaling function have also been developed. For example, Japanese Patent Laid-Open No. 2006-197573 (to be simply referred to as a reference hereinafter) discloses this technique. This reference or a technique based on this reference is called HD Photo® in general. More specifically, an image is segmented into tiles. Discrete Cosine Transform (DCT) is repeatedly performed for the respective tiles, thereby generating the stream data of layers. To suppress block distortion generated among the tiles, predetermined processing is executed using the pixels of two adjacent tiles before or during the DCT.
Some of the encoding methods with the resolution scaling function are difficult to use because of few scaling steps or a large resolution difference between scalable layers. For example, HD Photo enables direct decoding to three kinds of resolutions. To obtain image data at a desired resolution, image data obtained upon decoding processing must undergo a lot of processing.